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As the crew chief for the Derhaag Motorsports Corvette that was the fastest qualifier at Daytona this year (yet started seventh... go figure), I may be able to shed some light on this subject. The car we ran at Daytona (chassis 006) was purpose built in house to Grand Am rules. The basic chassis is identical to our Trans Am cars, but has different uprights fitted for larger brakes in the front, with the same size brake components in the rear. Bodywork is identical save for rear wheel arches. All de rigeur endurance equipment is fitted (lights, refueling gear, airjacks etc).
Engine rules are quite different. Our Grand Am engine package is 373 ci with a 600 cfm carburetor, mandating a dry weight of 2700 lbs. Aluminum blocks are allowed in GA. Our endurance spec engines have cast iron blocks by GM Motorsport, and are not production Corvette based. Trans Am engines are 310 ci with a 750 cfm carburetor. Aluminum blocks are not legal in TA, but the GM "Shortdeck" blocks are allowed, and this is what most Trans Am Corvettes run currently. In this configuration, the dry weight of the car is 2650 lbs. All of our engines are built by CRD. To paraphrase Rolls Royce, TA engines make "adequate" power, while the GA engines have "more than adequate" power. Both cars have a live rear axle with a Detroit Locker diff mandated by the rules. The TA cars have an aluminum cased diff while the GA cars use a steel cased unit. The GA car is much faster than the TA car owing to the larger brakes, larger tires, more power and slightly more downforce. Relative to the IMSA/ALMS cars the TA/GA cars have much less downforce and more drag as well, and this is where the much of the difference in laptimes shows up at Road America which places a premium on good L/D.
Our qualifying time at Daytona of 1:49.0 with Justin Bell at the helm (on race gearing and a "slightly higher than race" rev limit) stands up well to the times posted by the Pratt & Miller cars. I would love to see tubeframe cars allowed in ALMS to give a more direct comparison. I think some people would be very surprised...
I hope this helps.
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